1. Field of the Invention
The present invention relates to a display device and a method of repairing the same. More particularly, the present invention relates to a display device and a method of repairing the same which is preferably applied to a method of repairing a defective pixel in a semi-transmission type display device.
2. Background Art
Recently, flat panel displays (FPD) such as liquid crystal displays have been widely applied to monitors for personal computer use, monitors for television use and monitors for portable terminal use. Especially, in the field of mobile phones, portable type terminal devices have made a rapid progress. It is important that the portable type terminal devices are compact and capable of driving over a long period of time. In view of the above circumstances, a semi-transmission type display device is watched with keen interest, which is capable of saving electric power consumption in such a manner that the display device displays an image with a back-light as usual under a dark environment and that the display device displays an image by utilizing a peripheral environment light (external light) without using the back-light under a bright environment. For example, a semi-transmission type liquid crystal display device and a method of manufacturing the same is disclosed in Japanese Patent Publication No. 2003-255331.
The semi-transmission type display device includes a reflecting plate (reflecting electrode) in one portion of its display area so that light sent from the peripheral environment can be reflected by the reflecting plate. In many cases, this reflecting plate comprises the same thin metallic film arranged in a pixel as that of a scanning line or a video signal line. A thickness of a region, in which the reflecting plate is arranged, is larger than a thickness of a region, in which the reflecting plate is not arranged, in the direction of height. Therefore, a gap (referred to as “a panel interval” hereinafter) between an insulating board (array board), on which the scanning line and the video signal line are formed, and a color filter board, which is arranged being opposed to the array board, is reduced. When the panel interval is reduced and when foreign objects generated in the manufacturing process are attached to a portion between the panels before the process in which the array board and the color filter board are bonded to each other, defects of short-circuit are increased because of the existence of the foreign objects between the panels. In the case where a short-circuit is caused and a pixel electrode on the array board and an opposing electrode on the color filter board are electrically continued to each other, defects of luminescent spots are caused on the display having a structure of the normally white mode. Accordingly, for the above structural reason, the semi-transmission type display device is disadvantageous in that defects of luminescent spots, which are caused by the existence of foreign objects, are increased.
Recently, high quality is required for the product of a display device. Especially, when defects of luminescent spots are generated in the display device, it will be immediately rejected as a defective product. In order to repair the above defect of the luminescent spot, a first method of repairing the display device is provided, in which repairing is conducted by connecting a picture electrode with a gate electrode (a scanning line). According to this repairing method, a repaired pixel is given a predetermined gate-off potential. Therefore, it is expected that the repaired portion becomes a black spot. An example of this method is disclosed in Japanese Patent Publication No. H05-249488.
In order to repair the above defect of the luminescent spot, a second method of repairing the display device is provided, which will be described as follows. Recently, investigations have been made into a structure in which even when a defect of short-circuit is generated because of the existence of foreign objects, only the short-circuited region can be separated from the other pixel electrode. Especially, in the semi-transmission type display device, patterning is conducted in some cases so that a pixel electrode on the uppermost layer in the reflecting region, in which there is a high possibility that defects of short-circuit are generated by foreign objects because the panel interval is small, can be separated. Due to the above stricture, even when foreign objects are attached onto the reflecting region, the panel interval of which is small, and the array board and the color filter board are short-circuited to each other, it is possible to separate the pixel electrode of the reflecting region from the other region (the transmitting region) by cutting off the defective portion.
When the reflecting region, in which foreign objects exist, is repaired, the reflecting region remains as a luminescent spot. However, with respect to the transmitting region, it is possible to normalize it, which means that the reflecting region is visualized as a luminescent spot in the reflection display state, however, the reflecting region is not visualized as a luminescent spot in the transmission display state. As compared with the luminescent spot in the reflecting display state, the luminescent spot in the transmitting display state is a serious defect. Therefore, when the problem of the luminescent spot in the transmitting display state is solved, the number of defective products can be greatly decreased.
However, in the first method of repairing the display device described above, depending upon the V-T characteristic (Voltage-Transmission characteristic) of the liquid crystal to be used, when OFF voltage of the gate signal is impressed upon the liquid crystal, the transmittance is raised in some cases. As a result, the following problem is encountered. It is impossible to change the repaired pixel into a perfect black spot, that is, the repaired pixel can be only changed into a defective spot having some luminance.
In the second method of repairing the display device described above, the repairing work of repairing the defect is usually conducted after the panels have been assembled (after the array board and the color filter board have been bonded to each other) in many cases. In this case, it is necessary to irradiate a laser beam from a reverse side of the array board (a side opposite to the face opposed to the color filter board on the array board). However, the following problems may be encountered in the irradiation of the laser beam. In general, the pixel electrode includes ITO (Indium Tin Oxide) which is a transparent conductive film. However, the transparent conductive film is characterized in that a laser beam transmits through the transparent conductive film. Accordingly, in order to cut off the transparent conductive film, it is necessary to irradiate a very high intensity of laser beam. The very high intensity of laser beam penetrates the transparent conductive film and reaches the color filter board which is arranged being opposed. Therefore, the counter electrode formed on the face opposed to the array board on the color filter board, the laser beam machining of which is not necessary, is damaged. When the counter electrode is damaged on the color filter board, by an influence of the scattered scraps (foreign objects), the array board and the color filter board are short-circuited to each other, and a defect is generated again. When a black matrix portion, which is a shading region of the color filter board, is damaged, light leaks out from the thus damaged shading region at the time of lighting, which can be a cause of a defective product.